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The scientists said the evidence came after two landings by unmanned Viking spacecraft in 1976.

The space probes collected soil samples from the surface of Mars about 4,000 miles apart.

The soil was analyzed back on Earth. Some scientists said at the time that the samples contained signs of microbial life. Other scientists dismissed the claims, saying there were other explanations for the findings.

The matter has now been reexamined, and the University of Arizona and the National Institute of Health has published a paper that backs the original assertion.

The finding was that the possibility of life on Mars must be considered.